Field of the Invention
The invention relates to an atomic layer deposition (ALD) device and a process thereof for achieving high-quality deposition with high productivity and low gas consumption in the production of electronic devices, such as semiconductors, flat panels, solar cells, and light emitting diodes (LED).
Description of Related Art
Since the atomic layer deposition (ALD) technology was invented as an epitaxial technique by the group of Tuomo Suntola in the 1970s, it has been widely used for the production of electronic devices, such as semiconductor, flat panel, solar cell, and LED, as a high-quality deposition technique to achieve low impurity concentration and highly uniform coverage (step coverage) at low temperature. Especially, as described in Non-patent Literatures 1-4, because high step coverage is required for depositing high dielectric thin films of Al2O3 or HfO2 in formation of dynamic random access memory (DRAM) capacitors having a high aspect ratio shape where the depth is large with respect to the opening diameter, ALD has become a critical technology that determines the device performance. ALD is also expected to become the key for developing the double pattern technology which can improve the resolution limit capacity of the ArF immersion exposure apparatus as an alternative to the very expensive extreme ultra-violet (EUV) exposure apparatus. Here, a silicon oxide film, which has a thickness of 10-20 nm and has excellent uniformity and favorable step coverage with no dependence on the pattern shape, needs to be deposited on the patterned resist film at a low temperature of about 200° C. or less, which does not deteriorate the organic resist film, and as disclosed in Non-patent Literature 5, the ALD technology of SiO2 film is actively developed for mass production as the solution. Furthermore, formation of high-k/metal gate with use of the ALD technology has become an indispensable technique for manufacturing low power consumption semiconductors that are to be installed in portable terminals or tablet PCs. In semiconductor manufacturing, besides these processes, a number of ALD applicable processes have been developed, such as formation of DRAM capacitor upper and lower electrodes using metals, e.g. TiN and Ru (Non-patent Literatures 6-10), formation of gate electrode side wall using SiN, formation of barrier seed in contacts and through holes, and formation of high-k insulating film and the charge trap film for NAND flash memory, and are expected to become the key technologies in the future. In the areas of flat panels, LEDs and solar cells, research and development works for applying ALD, which achieves high step coverage and low-temperature processing, to ITO film formation or passivation film formation have also begun.
Single-substrate ALD equipment and batch furnace equipment have been the mainstream of ALD apparatuses. Especially, for forming high-k/metal gate, it is necessary to sequentially perform plasma oxynitridation of the silicon substrate or metal deposition in the same vacuum atmosphere as high-k ALD. Thus, the single-substrate ALD apparatus is widely used as one chamber installed in the cluster type system. In contrast, the batch furnace ALD equipment is most popular for forming the high-k insulating film of DRAM capacitor.
However, both the single-substrate ALD equipment and the batch ALD equipment have much lower productivity with a throughput of about 20 WPH (wafers per hour) or less, compared to other film deposition techniques. It is attributed to the essential defect of switching ALD, which alternately introduces gases into the process chamber by opening and closing the respective gas valves such that the metal-containing material gas and the non-metal gas, such as ozone, are not mixed in the reaction container, so as to deposit multiple atomic layers. For example, in the single-substrate ALD deposition of TiN, one ALD cycle includes 8 steps, which are vacuum evacuation, Ti precursor feeding, vacuum evacuation, purge gas feeding, vacuum evacuation, NH3 feeding, vacuum evacuation, and purge gas feeding. In each step, the valve switching, the process chamber pressure stabilization, and each reaction take about 1 second in total. Therefore, several seconds or more are consumed for completing one ALD cycle. As disclosed in Non-patent Literature 1, the amount of increase of the TiN film thickness in one single ALD cycle is ¼ TiN layer, which is about 0.1 nm, due to the steric hindrance among the precursors. Accordingly, when using the single-substrate ALD, it takes hundreds of seconds to form the 10 nm thick TiN layer. Thus, the throughput per process chamber is only about 4 WPH. Assuming that 4 process chambers are equipped on one deposition apparatus system, the total throughput can be only 16 WPH. Batch ALD, on the other hand, may contain about 100 substrates in one process chamber furnace. However, as the volume of the chamber furnace is increased dozens of times or more, the time required for gas supply and vacuum evacuation also becomes dozens of times more, and one ALD cycle would usually take several minutes. If the gas supply and vacuum evacuation are performed at a speed higher than this, gas replacement may be incomplete and particles may be generated by the reaction of the gas mixture. Then, it would take several hours to deposit 10 nm thick TiN films even with the batch ALD using furnace, which results in the throughput of about dozens of substrates per hour, almost equivalent to the single-substrate ALD system.
In order to enhance the throughput of these switching ALD processes, stacked or rotating semi-batch ALD devices have been in development in recent years. The stacked ALD, as seen in Patent Literature 1 or 2 for example, is a method of stacking several single-substrate ALD process chambers in the vertical direction and switching all the chambers at the same timing. Even though the stacked ALD improves the throughput several times compared to the single-substrate ALD chamber, it has not been used widely in the industry because of the high equipment manufacturing costs. The rotating semi-batch ALD device, on the other hand, includes a rotating table which circumferentially carries a plurality of substrates being processed, and a plurality of reaction gas and purge gas supply means which are disposed in a sector shape in an upper space of the rotating table. Each substrate is exposed sequentially to the gases through rotation the rotating table, so as to carry out the ALD processes. As the rotating ALD requires no gas switching that accompanies the gas supply and vacuum evacuation, which are essential for the conventional switching ALD, high-speed deposition is possible. Moreover, since the switching gas valves are unnecessary, the equipment manufacturing costs may be reduced. Now development of mass production of such rotating semi-batch ALD is in progress.
The first patent application for a rotating semi-batch ALD device was filed in 1990 (Patent Literature 3), in which a large cylindrical vacuum container is divided into four sector-shaped sub-chambers that are two reaction gas chambers and two purge gas chambers disposed between the reaction gas chambers, and a reaction gas supply means is disposed above the central part of each sub-chamber and a gas exhaust part is disposed at a lower part of the two purge gas chambers. By rotating a disk-shaped table, a plurality of substrates being processed on the table pass through the respective chambers for carrying out ALD deposition. This invention became the prototype of the rotating semi-batch ALD devices thereafter. In this configuration, however, each sub-chamber has a large volume and the gas supply part is disposed only at the central part of each sub-chamber. Therefore, the flow of the reaction gas and the purge gas becomes uneven or locally stagnant. The uneven or stagnant gas flow may result in insufficient ALD reaction and poor step coverage, and the residual gas may be mixed with other reaction gases and cause generation of particles.
Since then, various improvements have been made to the rotating semi-batch ALD and may be categorized into four types according to the reaction gas separation methods. The first type is a partition wall separation ALD device, in which the vacuum container is divided into relatively large sub-chambers by partition walls, the same as Patent Literature 3, and the gas supply/exhaust method is improved to make the reaction gas or the purge gas flow evenly in each sub-chamber. Patent Literatures 4-7 provide specific examples. In Patent Literature 4, a vacuum evacuation chamber is arranged between the reaction gas sub-chambers, such that the reaction gas flows uniformly from the reaction gas sub-chambers toward the vacuum evacuation chamber. In Patent Literature 5, a purge gas chamber is disposed between the reaction gas chambers to cause the purge gas to flow from the center to the peripheral direction, so as to enhance the separation of the reaction gases. In addition, in Patent Literature 6, the purge gas is ejected from the partition walls that surround the reaction gas sub-chambers to improve gas separation. However, these partition wall separation ALD devices are unable to sufficiently reduce uneven flow or stagnant reaction gas because of the large volume of the sub-chambers. Thus, it is difficult to completely solve the problems of low coverage and particles in Patent Literature 1. In Patent Literature 7, the vacuum container is divided into four sub-chambers and the gas supply means connected to each sub-chamber is provided with a switching function to supply the reaction gas or the purge gas in a pulse-like manner, by which one sub-chamber may be used as the reaction chamber or the purge chamber. With this configuration, the gas separation performance is improved and the types of the reaction gases may be varied to achieve deposition of multiple atomic components. Nevertheless, since it is required to perform gas exhaust and supply following the gas switching, the ALD cycle time becomes very long.
The second type of the rotating semi-batch ALD devices is a gas curtain ALD device, in which the purge gas flows like a curtain between the reaction gas supply means to reduce mixture of the reaction gases. Patent Literatures 8-11 provide specific examples. In Patent Literatures 8 and 9, the purge gas is streamed downwards from the top to prevent mixture of the reaction gases. In Patent Literature 10, the purge gas flows upwards from the bottom to the top to form the gas curtain. Moreover, in Patent Literature 11, a reaction gas nozzle and a purge gas nozzle are arranged alternately and radially and rotated, so as to alternately supply the reaction gas and the purge gas to each substrate. Such gas curtain ALD has the advantage of simpler structure in comparison with the partition wall separation ALD device, but is inferior in gas separation and thus particles caused by mixture of the reaction gases may occur easily.
The third type of the rotating semi-batch ALD devices is a micro-reactor ALD device, where the functions of gas supply and exhaust are integrated into a compact rectangular reactor having a width of about several centimeters, and by preventing the reaction gas from leaking outside the reactor, the gas separation function is enhanced. A plurality of such reactors are disposed and different reaction gases or purge gases are supplied to the respective reactors, and the gases are exhausted by the adjacent part so as to achieve favorable gas separation performance. Patent Literatures 12-15 provide specific examples. Patent Literature 12 was filed in 1979 and became the prototype of such micro-reactor, which discloses a structure that a first reaction gas nozzle, a first purge gas nozzle, a second reaction gas nozzle, and a second purge gas nozzle are arranged linearly through the respective exhaust ports. Patent Literature 13 adopts a structure in which a plurality of rectangular micro-reactors with one single gas supply and exhaust function are disposed radially in the vacuum container. By using the reaction gas as an etching gas, not only deposition, etching, pre-deposition treatment, or film modification after deposition is also possible. In Patent Literature 14, a first reaction gas nozzle, a first purge gas nozzle, a second reaction gas nozzle, a second purge gas nozzle, and the exhaust ports disposed between the nozzles constitute one compact sector-shaped ALD reactor, and a plurality of such ALD reactors are arranged radially and sequentially. Moreover, in Patent Literature 15, the first reaction gas is charged into a large vacuum container, and the second reaction gas and the purge gas are supplied and exhausted from the rectangular micro-reactors that are disposed radially. This configuration does not require the micro-reactor of the first reaction gas and simplifies the structure. These micro-reactor ALD devices may significantly improve gas separation compared to other types of rotating ALD devices.
However, with the micro-reactor structure, the trade-off between productivity and the step coverage becomes more serious, compared to other ALD types due to the narrower width of the micro-reactor. For example, in the case where the width of one micro-reactor is several centimeters, even if the rotation speed of the table is as low as about 30 RPM, the exposure time to the reaction gas is as short as dozens of milliseconds, which is too short to achieve sufficient ALD reaction and consequently the step coverage is low. Particularly, for the second reaction gas that does not include a metal, because the ALD deposition usually requires more than 100 ms, the short exposure time is a big problem. The rotation speed may be lowered to increase the exposure time and more micro-reactors may be arranged to improve the productivity, but the productivity drops instead. The reason is explained below. It is known that, in the ALD process, the relationship between the exposure time of the substrates being processed to the reaction gas and the deposition rate of one single ALD cycle is as shown in FIG. 1, wherein when the exposure time is short, the deposition rate increases linearly to the exposure time; and when the exposure time exceeds a certain time, the deposition rate is saturated and becomes a constant value. This saturation phenomenon corresponds to a state where all the adsorption reaction sites on the surface of the substrates being processed are covered with the reaction gas. The time when the deposition rate begins to saturate may be defined as an ALD saturation reaction time and the region where the deposition rate is saturated and becomes constant may be defined as an ALD saturation reaction region. In the rotating semi-batch ALD device, if the number of the substrates being processed on the rotating table is n, the number of the micro-reactors is q, and the rotation speed of the rotating table is r rpm, when depositing a film with a thickness of A nm at the deposition rate of a per minute in one single ALD process (nm/cycle), the throughput W is calculated by W=60 nqra/A (WPH). When the rotation speed is sufficiently low and the ALD reaction falls in the saturation reaction region (equivalent to the point A in FIG. 1), the throughput is proportional to the product of the number n of the substrates being processed and the number q of the micro-reactors according to the above equation. Therefore, the throughput is improved as the number of the substrates being processed on the rotating susceptor and the micro-reactors increases. However, under the condition that the exposure time usually used in the micro-reactor is dozens of milliseconds, the reaction is in operation between the ALD unsaturation reaction region (the point C in FIG. 1) and the limit region where the ALD saturation reaction is obtained (the point B in FIG. 1). In the ALD unsaturation reaction region, the exposure time is reduced as the number of the micro-reactors q increases, and consequently the ALD deposition rate a decreases. For this reason, the throughput W does not increase even though the number of the micro-reactors q is increased. Each micro-reactor requires a minimal gas separation. As the number of the micro-reactors q increases, the area that the gas separation region occupies in the reaction gas supply unit is relatively increased. Correspondingly, the time of contact with the reaction gas is shortened. Therefore, the trade-off between the throughput and the step coverage becomes more serious. For the rotating semi-batch ALD device, it is advantageous to dispose fewer gas supply units that occupy a relatively large area, rather than arranging more micro-reactors, in view of the trade-off between productivity and step coverage.
The fourth type of the rotating semi-batch ALD devices is a narrow gap gas dispersion ALD device, which includes a pair of first and second reaction gas supply units and a purge gas region disposed between the reaction gas supply units, wherein a dispersion plate is attached to the gas supply nozzle, and a shower plate is used to make the reaction gas and the purge gas flow uniformly in a narrow gap space between the gas nozzle and the substrate. This configuration makes it possible to perform operations at a higher rotation speed than the micro-reactor configuration and the productivity is expected to improve. The series of inventions of Patent Literatures 16-21 and Patent Literature 22 provide specific examples. In the series of inventions of Patent Literatures 16-21, the gas dispersion plates are arranged adjacent to the reaction gas and the purge gas. Also, various works are made to improve the gas flow uniformity by setting the purge gas temperature to a high temperature, increasing the circumferential length of the second reaction gas dispersion plate relative to the circumferential length of the first gas reaction gas dispersion plate, arranging the gas nozzle ejecting hole to face forward, disposing the purge gas rectification plate close to the reaction gas nozzle, optimizing the position of the exhaust hole, and so on. Moreover, in Patent Literature 22, sector-shaped shower plates are used to replace the gas nozzles of the reaction gas and the purge gas, and a gas exhaust region that is narrow in width is disposed between the shower plates and the gases are exhausted from a few exhaust holes. Such narrow gap gas dispersion ALD configurations yield high throughput but are disadvantageous in significantly increasing the gas consumption to about dozens of SLM, which raises the consumption material costs. In addition, these gas dispersion configurations have a drawback that the reaction gas utilization efficiency is as low as 0.5%, and more than 99.5% of the reaction gas is discharged without being used.
Furthermore, the high gas flow induces another problem. That is, when a large gas flow is generated in one direction on the substrates being processed, a pressure difference occurs between two ends of the substrate and causes the substrate to float up. This problem may easily cause the substrates being processed to float especially when forming a film with high compressive stress. If the substrate floats, the substrate may hit the upper gas supply part and be damaged. Thus, when the gas flows at a high flow rate in one direction on the substrates being processed, the process reliability of the manufacturing device is seriously affected.
Moreover, according to the inventions of the narrow gap gas dispersion ALD devices as disclosed in the series of Patent Literatures 16-21, the table that holds the substrates being processed is rotated at a high speed of about 300 RPM, in order to achieve high productivity. Due to the high-speed rotation, the exposure time of the substrates to the reaction gas is 100 ms or shorter, which results in the state that does not reach the ALD saturation reaction region, as indicated by the point C in FIG. 1. In such a case, the problem of low step coverage becomes worse for trenches or holes with a high aspect ratio.